1. Technical Field
The present invention relates to DC (direct current) motors having brushes for supplying power to the DC motors.
2. Description of the Related Art
Up to now, DC motors having brushes have been used for various machines and apparatus, e.g., vehicles, because the DC motors have a relatively high efficiency and can be easily controlled.
FIG. 4(A) schematically illustrates a known DC motor that has been formed by using a concentrated winding method. The known DC motor is configured as a four-pole and six-slot type of DC motor that has four permanent magnets and six slots, i.e., six coils (delta connection type) and six cores around which the coils are wound. Each of the permanent magnets has an N-pole surface and an S-pole surface. The terminals of each coil are connected to two corresponding commutator segments. In general, the number of coils is set to be larger than the number of poles (permanent magnets) in order to help minimize dead points during rotation of the armature.
More specifically, in the known DC motor shown in FIG. 4(A), permanent magnets M1–M4 are fixed to an inner wall of a yoke in order to constitute a stator. The quantity of individual permanent magnets is chosen to be even in number. For example, in the known DC motor four poles (permanent magnets) M1 to M4 are arranged in a circle to substantially form a cylinder. The polarity of the surface of each of the poles M1–M4, on the side facing the rotor, alternates from magnet to adjoining magnet. Thus, the polarity of a surface directly opposing the rotor of the magnet M1 is N, and the polarity of same surfaces of the permanent magnets M2 and M4, both of which adjoin permanent magnet M1, is S.
The rotor includes cores T1–T6 with slots R1–R6 formed therebetween. Coils C1–C6 are wound around the corresponding cores T1–T6 via the corresponding slots. Commutator segments S1–S6 are connected to the terminals of the corresponding coils. The commutator segments S1–S6 are electrically insulated from each other. The rotor rotates about the center axis that defines the stator. The rotor rotates within the stator.
Brushes B1–B4 are disposed and are arranged such that the individual commutator segments S1–S6 form an electrical connection with various individual brushes B1–B4 when the commutator segments S1–S6 rotate through a predetermined angle. The number of the brushes B1–B4 is the same or smaller than the number of the poles (permanent magnets) in the configuration shown in FIG. 4(A), the number of the brushes (four) is equal to the number of the poles. A power source E is connected to each of the brushes B1 to B4 via a positive or negative terminal, so that the brushes supply current to the coils via the commutator segments that contact the brushes.
FIG. 4(B) illustrates an example of positional relations between the brushes B1–B4 of the DC motor shown in FIG. 4(A). In this example, the brushes B1 and B3 each extend approximately 20° around the center axis of the armature. Brush B1 begins from approximately a 25° position and extends in a counterclockwise direction (assuming the horizontal axis to the right of the rotor to be a 0° reference position). Brush B3 begins approximately 25° below a 180° reference position (205° from the 0° reference of B1) as shown in FIG. 4(B). Thus, each of the brushes B1 and B3 extends from an initial position indicated by DegB1s to a final position indicated by DegB1e. Similarly, the brushes B2 and B4 extend over a range of 20° from positions rotated 25° counterclockwise respectively from a 90° reference and 270° reference positions (105° and 295° from the 0° reference of B1) as shown in FIG. 4(B). Thus, each of the brushes B2 and B4 extends from an initial position indicated by DegB2s to a final position indicated by DegB2e. 
In the case of the DC motor shown in FIGS. 4(A) and 4(B), the rotor is rotated in the counterclockwise direction. The current is supplied to the coil C1 during the time when the boundary element between the commentator segments S1 and S2 is not within the 20° range corresponding to the contact area of any of the four brushes B1–B4. Thus, during each rotation of the rotor, the coil C1 has four separate current supply periods. The supply periods correspond to when the S1 and S2 boundary element is rotated within the range established between DegB1e and DegB2s and the range established between DegB2e and DegB1s. The same general principle also applies to the other coils C2–C6.
Next, the magnetic flux density around each coil during one current supply period will be described with reference to FIGS. 5(A) and 5(B).
FIG. 5(A) illustrates coil C1 having a boundary element positioned within an angular range between DegB1e (45° from the previous explanation), and the boundary between pole M1 and pole M2 (90°, as shown in FIG. 5(A)). In this state, a current is supplied to the coil C1 to produce a magnetic field (φc) within the corresponding core T1. The magnetic field (φc) thus produced extends in a direction away from the center of rotation of the rotor towards the pole M1. Simultaneously, a magnetic field (φm) is produced within the core T1 by the poles M1 and M2. The direction of the magnetic field (φm) produced by the poles M1 and M2 is inverse to the direction of the magnetic field (φc) produced by the electric current. The magnetic flux density generated by the current induced magnetic field (φc) around the coil C1 is reduced by the magnetic flux density of the magnetic field (φm) generated by the permanent magnets. Therefore, in the following descriptions, this angular range is called the “reduced density area.”
FIG. 5(B) illustrates the coil C1 positioned within an angular range from the change in polarity of the poles M1 to M2 (90° as shown in FIG. 5(B)), to a position where the supply of current for coil C1 is terminated at DegB2s (105° from the 0° reference). In this range, a current is supplied to the coil C1 to continue to produce a magnetic field (φc) within the corresponding core T1. The magnetic field (φc) thus produced, as explained previously, extends in a direction away from the center of rotation of the rotor towards the pole M2. Similar to the situation described in FIG. 5(A), a magnetic field (φm) is also produced within the core T1 by the poles M1 and M2. However, in this case, the direction of the magnetic field (φm) produced by the poles is the same as the direction of the magnetic field (φc) produced by the supplied current. Therefore, the magnetic flux density around the coil C1 is increased by an amount equal to the magnetic flux density of the magnetic field (φm) produced by the permanent magnets. The magnetic flux density around the coil C1 is approximately equal to an amount produced by the total magnetic field (φm+φc). In the following description, this part of the angular range is called an “increased density area.”
FIG. 6 illustrates an example of a counter electromotive voltage that is superimposed onto a power source voltage from a power source E. The counter electromotive voltage is generated when the known DC motor described with reference to FIGS. 4(A)–4(B) and FIGS. 5(A)–5(B) has been started. As shown by this example, the change in the counter electromotive voltage per unit angle of rotation of the rotor (ΔVz/Δφz) increases significantly when the supply of current to each coil has been interrupted, i.e., when the boundary element between two adjacent commutator segments is within the range defined by the brushes B1–B4. In case of the DC motor shown in FIGS. 4(A)–4(B) and FIGS. 5(A)–5(B), the supply of current to any of the individual coils is interrupted at intervals of 30°. The counter electromotive voltage shown in FIG. 6 therefore also appears at a 30° cyclic period.
As will be seen from FIG. 6, the rate of change of the counter electromotive voltage per unit angle of rotation of the rotor (ΔVz/Δφz) increases when the supply of current to the coils is interrupted. If the rate of change of the counter electromotive voltage increases, a relatively high possibility of undesirable discharge exists between the brushes and the commutator segments, causing excessive wear of the brushes.
In order to teach a relatively low cost method to lessen the reduction of the overall magnetic density due to the interaction between the permanent magnets and the coil, Japanese Laid-Open Utility Model Publication No. 57-197779 proposes a magnetic field assembly using a reduced density portion of the pole having a smaller thickness than the main pole part.
Further, Japanese Laid-Open Patent Publication No. 57-59465 teaches a DC motor in which a high magnetic permeability material is combined with a combination of a permanent magnet having a good property against the reduction of magnetic density and a permanent magnet having a large magnetic flux in order to provide a property similar to a series wound DC motor.
However, a recent search produced no prior art that specifically teaches countermeasures against the increase in the magnetic flux density. Therefore, it is not considered easily possible to reduce or minimize the magnitude of the change of the counter electromotive voltage per unit angle of rotation of the rotor (ΔVz/Δφz). As a result, potential discharges between brushes and commutator segments may have not been prevented, and excessive wear of the brushes may have not been reduced.